Known sensor assemblies typically comprise composite ceramic/metal components that are brazed together using a conventional brazing process. Such a known sensor assembly might include a metal housing with a metallised aluminium oxide bush brazed into the inner diameter of the housing. A sensor body is then brazed into the internal diameter of the bush.
The sensor body can be made of one or more layers of metal, electrically conductive ceramic, electrically non-conductive ceramic that is made conductive by having a layer of conductive material (e.g., a metal) deposited on its surface, or a conductive ceramic/metal composite, for example. Conductive layers can define electrodes or other sensing elements or shield layers. Non-conductive layers can define insulating spacers that are positioned between conductive layers. The layers that form the sensor body can be machined as a preformed part and then bonded to an adjacent layer or deposited on an adjacent layer using any suitable deposition process. If the outer layer of the sensor body is made substantially from a ceramic material, then its outer surface can be metallised so that the sensor body can be brazed directly into the housing using a conventional brazing process without the need for the intermediate bush.
The metal housing parts of the sensor assembly might be manufactured from a low expansion alloy which is specifically designed to have a coefficient of thermal expansion substantially similar to that of the bush and/or the sensor body. If the sensor assembly is exposed to high temperatures during operation then the housing, bush and sensor body all expand at similar rates to minimise the thermal stress between the individual components.
One problem with the use of low expansion alloys is that they tend to oxidise at temperatures approaching 500° C. This places an upper limit on the operating temperature of the sensor assembly. It can be difficult to find a metal that is suitable for use at higher temperatures and which also has a thermal expansion coefficient that is substantially similar to that of the bush and/or the sensor body. A known solution is to use so-called ‘active braze’ processes which allow certain ceramic materials to be brazed to metals without the need for metallised coatings and also provide a degree of compliance between the two different materials to accommodate the different rates of thermal expansion. In practice, however, the operating temperature of active braze alloys is limited to about 800° C. which is still not sufficiently high for certain operations. The compliant coatings that are needed to provide the degree of compliance also tend to oxidise at temperatures below 500° C. and it is normally necessary to provide a hermetic seal at the braze interface to minimise the oxidation effect.
Further problems are known to exist in situations where large relative movements occur between the component parts of the sensor assembly as a result of thermal expansion. Large relative movement can only be accommodated by increasing the thickness of the compliant coatings and this can place practical limitations on the design of the sensor assembly.
In many industrial measurement applications there is a need for a sensor assembly that can be used at high operating temperatures to measure the distance to either a stationary or passing object. A typical application is the measurement of clearance between the tip of a gas turbine engine blade and the surrounding casing. In this situation the operating temperature of the sensor assembly can reach 1500° C. Other applications including molten metal and molten glass level measurement, for example, have similar operating temperature requirements.
U.S. Pat. No. 5,760,593 (BICC plc) and U.S. Pat. No. 4,804,905 (Ding et al.) describe sensor assemblies having an electrode, optionally in the form of a metal coating or layer, that couples capacitively with the stationary or passing object. The electrode is connected to the centre conductor of a standard triaxial transmission cable and is surrounded by a metal shield and a metal outer housing. The shield and the outer housing are connected directly to the intermediate conductor and the outer conductor of the triaxial transmission cable respectively. An insulating layer is provided between the electrode and the shield and also between the shield and the outer housing. The insulating layers can be in the form of machined ceramic spacers or deposited ceramic layers, for example.
One problem with these conventional sensors is that they typically utilise an alternating combination of metal and ceramic materials. As the operating temperature of the sensor assembly increases, the metal components tend to expand more than the ceramic components. This often results in stress fractures forming in the ceramic spacers or layers, which reduce their electrical performance and may even result in the disintegration or de-lamination of the ceramic components. Not only does this cause the sensor assembly to fail electrically, but the disintegration or de-lamination of the ceramic components also allows the metal components to vibrate and this can result in the mechanical failure of the complete sensor assembly. A similar problem can occur if electrically conductive ceramics are used since just a small difference in the respective coefficients of thermal expansion (CTE) can be significant over the expected lifetime of the sensor assembly.
Gas turbine engine manufacturers now require an operating lifetime of at least 20,000 hours for sensors that are to be fitted to production models. Although conventional sensors have been successfully used at high operating temperatures for short periods of time, it is unlikely that they will ever be able to meet the required operating lifetime because of the inherent weakness of the sensor assembly caused by the different thermal expansion properties of the metal and ceramic (or ceramic and ceramic) components.
Conventional sensor assemblies are also susceptible to moisture penetration which can reduce the performance of the sensor.
International Patent Application WO 2012/049443 (Future Technology (Sensors) Ltd) describes a sensor body with an electrically conductive electrode layer between a core layer and an insulating layer. The electrode layer may be exposed at a rear face of the sensor body to allow it to be connected, either directly, or indirectly by means of an intermediate electrically conductive metal bridge, to an inner conductor of a coaxial or triaxial transmission cable. The metal bridge is typically brazed to the sensor body. But such brazed layers formed when ceramic and metal components are brazed together can, in some circumstances, restrict operating lifetime and temperature due to the difference in CTE. This can sometimes cause failure of the brazed layer after repeated thermal cycles.